<p>Small esters, including methyl lactate (MELAC), and ethyl lactate (ETLAC), have recently been recommended as potential oxygenated additives and biodiesel precursors. In this study, controlled thermal decomposition experiments of MELAC and ETLAC were conducted using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVU-PI-MS). To reveal the pyrolysis mechanism of MELAC and ETLAC, reactive force field (ReaxFF) molecular dynamics (MD) simulations were employed to investigate the thermal decomposition of MELAC, ETLAC, and mixture fuels at elevated temperatures (1800 K–2400 K). The reaction rate coefficients of fuel pyrolysis were fitted by Arrhenius equation according to present simulations, and ETLAC exhibited higher reactivity than MELAC that was attributed to its larger carbon chain and molecular structure. From the perspective of species distributions, the final decomposition products were primarily CO₂, C₂H₄, and small molecular fragments. The decomposition of MELAC was initiated by a cleavage of C-O bond reaction (MELAC=CH₃CHOHCOȮ+ĊH₃), while ETLAC underwent the reaction of ETLAC=CH₃CHOHCOȮ+Ċ₂H₅. During thermal decomposition, the shortening of carbon chains followed two distinct pathways: (C₄-ĊH₃) → (C₃-CO₂)→C₂ and (C₅-Ċ₂H₅)→(C₃-CO₂)→C₂. Compared to MELAC, ETLAC can produce more C₂H₄ via Ċ₂H₅ and ĊH₂CH₂OH radicals, whose curve was divided into two stages of rapid rising and slow equilibrium. Additionally, a competitive reaction channel of MELAC=CH₃Ȯ+CH₃CHOHĊO became more important in the mixed fuel, and the generation of CH₃Ȯ also gives a reasonable explanation to the less soot tendency of MELAC fuel. This work provides a theoretical foundation for reducing carbon emissions by utilizing ester-based biomass fuels.</p>